Surveying and registering a densely located system of public utilities raises many problems. For example, with previous methods, determination of the horizontal position and depth of buried metallic conductors can only be accomplished with a relatively low accuracy, especially in a very "noisy" environment, i.e., in the vicinity of electrical power lines and equipment, e.g., tramway and trolley lines. The handling of known devices is sometimes difficult. Portable devices are not available; and the result of the measurements may be deteriorated by significant human errors.
A general method of locating metallic conductors is to indicate the variations of the vertical and/or horizontal components of the magnetic field generated by an alternating current existing in said conductor and detected by means of a suitable probe, usually a transducer comprising ferrite antennas. An alternating current of the required amplitude and frequency may be produced by a generator and applied to the conductor to be located at its available points. An alternating current may alternatively be induced in the conductor to be located. A common method is to connect only one pole of the generator to the conductor, the other pole being grounded.
The locating accomplished by the method described is based on the fact that if the conductor to be located is assumed to be an infinitely long straight wire, then a concentric magnetic field will exist around it. In the case of such a field pattern, the vertical and horizontal components of the magnetic field strength are equal if the wire is located at an angle of 45.degree. from the point of measurement. The position of the conductor can be determined using this fact. Theoretically, the horizontal position of the conductor can be established by detecting the maximum value of the horizontal component and the zero level of the vertical component. Both measurements are, however, uncertain; the measurement of the maximum value may be burdened by serious human errors, and measuring the zero value is illusory, especially in a noisy environment. A device has been developed by the United States company Metrotech that utilizes a microcomputer to realize the locating method describd, wherein the measuring process is based on a stored program.
Circuits have been developed to accomplish the method described above, and are shown in German Patent Specifications DE-PS Nos. 10 64 655, 25 28 511 and 20 37 865. The solution of particular partial problems are given in these Patent Specifications to increase the accuracy or stability of detection or to accomplish an energy saving construction of the generator (powering the generator from the conductor to be located is recommended by one of these specifications).
An apparatus is described in German Patent Specification No. 955 348 that comprises two coils arranged horizontally, one above the other, and in a fixed position with respect to each other, to find the depth, while a separate antenna, which is also rigidly fixed in relation to the coils, is provided to determine the horizontal position of the buried conductor. This apparatus requires careful calibration when used.
In noisy environments, applying a generator raises particular problems, as 100 to 500 watt high-power units are used in an attempt to generate a sufficiently high level of alternating current in the conductor to suppress the disturbing effect of any current already present in the conductor. The power required to achieve this is very high, and thus developing mobile apparatus is difficult, and the generator must be carefully matched for the measurements.
A source of additional difficulty is the measuring frequency, the selection of which is commonly considered as incidental. The measuring frequency is typically an odd-order multiple of the mains frequency or approximates this value (e.g., 1450 Hz in the case of an instrument made by the SEBA company, or 1030 Hz in equipment made by RFT). In attempting to detect a magnetic field generated by an alternating current of such frequencies, the relative value of the noise may be significant, and thus a sufficiently selective measurement can not always be effected.